A steering of an automobile or the like in the related art is controlled at an optimal torque by an electronic control unit (micro-controller or micro-computer, hereinafter, referred to as the micro-controller) in consideration of a vehicle speed or the like. However, when a power-supply voltage of the micro-computer performing control goes out of a normal operation voltage range of the micro-controller and an abnormality occurs in an operation of the micro-controller, inconveniences, such as rotation and locking of the steering, may possibly occur. It is therefore necessary to maintain the power-supply voltage within an appropriate range and also to constantly monitor the power-supply voltage to be supplied to the micro-controller.
As a measure to prevent the occurrence of such an abnormality resulting from a fluctuation of a power-supply voltage of the micro-controller by monitoring the power-supply voltage, for example, a micro-computer power-supply voltage monitoring system is described in PTL 1. The micro-computer power-supply voltage monitoring system 1 is formed of a micro-computer 10 that controls an outside load 60 and a power-supply supply portion 20 that supplies the micro-computer 10 with a power-supply voltage. An operation monitoring circuit 30 (formed of a watchdog circuit and homework solutions) monitoring whether an operation of the micro-computer 10 is normal or not is connected to the micro-computer 10. A power-supply voltage supplied from the power-supply supply portion 20 is applied to a power-supply terminal of the micro-computer 10 and at the same time introduced into the micro-computer 10 from an input and output port thereof. The power-supply voltage inputted from the port is compared with an appropriate power-supply voltage value 11 indicating “a power-supply voltage range within which an operation is guaranteed” for the micro-computer 10, so that whether the power-supply voltage is an appropriate power-supply voltage or not is monitored. On the other hand, the micro-computer 10 controls an output driver 40 connected to the outside load 60 by outputting a control signal Sc. An output of this power-supply monitoring means 12 and an output of the operation monitoring circuit 30 are introduced into a logical operation circuit 50, such as an OR circuit. When it turns out that the power-supply voltage supplied to the micro-computer 10 is not the appropriate power-supply voltage or an operation of the micro-computer 10 is not normal, the control on the outside load 60 is stopped by inhibiting the output driver 40 that controls the outside load 60 or by resetting the micro-computer. The problem is solved in this manner.
Also, a scanner motor control apparatus described in PTL 2 is formed of a control circuit including a CPU that controls the driving and stopping of a motor 12 driving a document scanning scanner and is reset by a reset signal, and a monitoring circuit 10 that monitors a power-supply voltage Vcc and generates the reset signal when the power-supply voltage Vcc drops to or below a set value. The scanner motor control apparatus includes monitoring circuits 11 and 10 that generate, respectively, a first signal S1=L when the voltage Vcc drops to or below a first set value VREF1 and a second signal S2=L when the voltage Vcc drops to or below a second set value VREF2 smaller than the first set value VREF1. The CPU performs the stopping control in response to the first signal S1=L during the driving of the motor and is reset by the second signal S2=L. The first signal S1=L is applied to an interruption port INT B and the CPU 14 performs the stopping control by interruption processing. The first signal S1=L is applied to an interruption port assigned with the top priority. By applying the first signal S1=L to the interruption input port assigned with the top priority, the CPU performs the stopping control of an electric motor first of all when the power-supply voltage drops. This configuration lessens a burden of the monitoring of a signal level variance. Moreover, because a delay in recognition of a signal level variance is short, the stopping control in response to a signal level variance (from H to L) of the first signal S1 can be achieved more effectively.